Mineralogy and Petrology最新文献

The physicochemical evolution of cratonic lithosphere reflects the impacts of tectonomagmatic processes over its lifetime that may be deciphered using kimberlite-borne xenoliths and xenocrysts, but remain poorly constrained for the Indian Dharwar craton, owing to the dearth of fresh mantle material. This study examines detailed petrography and geochemical composition of six eclogite xenoliths, and additional eclogitic and peridotitic garnet separates, from the Wajrakarur kimberlites in the Eastern Dharwar Craton (EDC). Clinopyroxene in eclogite xenoliths is too altered to permit contamination-free sampling during laser ablation for trace element analysis. We overcome this limitation by exploiting relationships of clinopyroxene-garnet distribution coefficients with garnet Ca#, clinopyroxene jadeite content, and temperature. This allows a more accurate delineation of their petrogenesis from reconstructed bulk rocks and indicates their origin from variably plagioclase-rich oceanic crustal protoliths, with weak subsequent metasomatic overprint. In contrast, estimates of Fe³⁺ in garnet from peridotite xenoliths indicate an oxygen fugacity shift towards more oxidized conditions beneath the EDC linked to enrichment in melt-mobile elements (Ti, Zr) in the barren or weakly diamondiferous P1 and P3 kimberlites. The most depleted and reduced sample [ΔlogfO₂ (FMQ) of -4.3; where FMQ corresponds to the fayalite-quartz-magnetite buffer] derives from diamondiferous kimberlite P7, suggesting oxidative melt metasomatism as a key control on the regional diamond inventory, although more data are needed. EDC eclogites and peridotites have estimated P-wave velocities of 8.46–8.63 km/s and 8.21–8.22 km/s, respectively, which are lower than present-day observed bulk P-wave velocities, and may point to lithological or thermal changes since Mesoproterozoic entrainment.

A log-normal size frequency distribution (SFD) of a primary diamond deposit provides a useful basis for extrapolating and interpolating an anticipated size recovery derived from bulk samples, especially where a single dominant population is present. To assess a deposit for economic potential the run-of-mine (ROM) diamond value or price is a key consideration. Traditionally this value has been achieved by obtaining for valuation a sample that provides sufficient quantity of diamonds in larger sizes. An alternative approach combines a modelled SFD with a modelled price-size relationship, particularly of the carater sizes. It was found in this study that generally the average $/ct value of a diamond size fraction is linearly dependent on size for sizes above 3 grains (gr). It was also found that the value profile within a size class larger than 0.7 ct were very similar, allowing a transformation of values of single stones to that of an equivalent 3 gr (‘grain’ is equivalent to ¼ ct stone). Bayesian modelling showed that for a 5000 stone sample, a modelling approach was more accurate than the conventional method and much less sensitive to the inclusion of a single high value stone.

Diamonds from the Star kimberlite at Fort à la Corne formed in an unusual substrate: 94% of inclusion-bearing diamonds derive from refertilized cratonic lherzolites, with olivine Mg# [molar 100 Mg/(Mg + Fe)] centered around a mode at 88.7. In addition, there is a minor eclogitic suite (6%) and a single sublithospheric diamond is most likely linked to oceanic crust subducted into the lower mantle. In addition to low Mg#, the refertilized lherzolitic association is characterized by low Ni contents, elevated V and Ti at normal Na contents, garnet rare earth element patterns very similar to primitive mantle garnet, and positive Nb anomalies. These characteristics are best explained by refertilization through a kimberlitic low degree partial melt. Single clinopyroxene-based geothermobarometry for inclusions in diamond and kimberlite-derived concentrate yields identical cold geotherms (equivalent to ~ 37 mW/m² surface heat flow), which implies that diamond formation occurred in a steady state thermal environment that did not change measurably from the time of diamond formation (1.26 Ga) to the time of kimberlite emplacement (0.10 Ga). Consequently, the refertilization event affecting the lherzolitic diamond substrates must have predated diamond formation. A further unique signature of the lherzolitic diamond association is its carbon isotope composition, with 97% of diamonds having δ¹³C values between − 18.0 and − 14.6‰. This constitutes the first observation of a peridotitic diamond suite dominated by subducted carbon, originating as organic matter or biogenic carbonates, instead of mantle-like carbon.

Three oxide minerals belonging to the columbite group, including columbite–(Mn) (MnNb₂O₆), columbite–(Fe) (FeNb₂O₆), and fersmite (CaNb₂O₆), were found in specimens from gem placers in Ratnapura district, Sri Lanka. We present results of chemical, structural and spectroscopic analyses, including the first Raman spectrum for natural fersmite. For columbite–(Mn) and columbite–(Fe), analytical results obtained before and after dry annealing in air indicate low degrees of accumulated self-irradiation damage, which corresponds well with low actinide concentrations.

We report the major and trace element composition of high-density fluids (HDFs) trapped in microinclusions in 14 diamonds from Wafangdian, Liaoning, China. This is the first detailed report of the major and trace elements of HDFs from the North China Craton. The trapped fluids are similar to those known from other localities, except for one with a unique Cl-K-Ba-Sr-rich composition that is also extremely enriched in Th and the light rare earth elements (REEs). One diamond contains a saline HDF of typical composition. The other twelve diamonds exhibit high-Mg carbonatitic HDFs with trace element patterns that resemble those of the Wafangdian kimberlites, but with higher concentrations. The nitrogen aggregation level of all the diamonds is similar, with 6–37% of the nitrogen residing in B centers, suggesting mantle residence temperatures of 1100–1200 ºC. Comparing the composition of the carbonatitic HDFs to available experimental data on melting of carbonated peridotite, we show that they can be produced by a very low degree of partial melting (< 0.1%) of mantle peridotite. In an oxidized environment, such melts are stable in most of the lithospheric mantle and the main barrier for their existence is the reduced nature of the lithosphere. With increasing temperature, the experimental melts evolve towards kimberlites. The similar trace element patterns of the HDFs and their host kimberlite also suggest derivation from similar sources. Still, the difference in the formation temperatures means that the HDFs were trapped in the lithosphere, whereas kimberlite formation requires asthenospheric conditions. Crystallization of kimberlite at lithospheric levels face difficulties, but forming high-Mg carbonatitic melt at the top of the asthenosphere and its trapping in diamond after minimal crystallization in the lithosphere may solve them. More complex scenarios may be needed to explain the formation of both HDFs and kimberlites.

“Floating reefs,” also known as “mega-blocks,” occur within kimberlite and non-kimberlite maar-diatreme volcanoes. In most cases, floating reefs consist of large accidental blocks derived from the pipe wall country rocks. Some other floating reefs comprise blocks of solidified cognate material from earlier emplacement phases. In addition, we include isolated entities of unconsolidated sediments derived from the host substrate or the crater floor as floating reefs, since they are of no economic value for mining. In general, floating reefs usually occur along the diatreme wall and may be intact or partially or completely fragmented. Within the diatreme, they have subsided from higher stratigraphic levels and can be found at all structural levels of a maar-diatreme volcano. Floating reefs can be hundreds of meters in size and may have subsided over 1000 m. In both kimberlite and non-kimberlite diatremes, the presence of floating reefs suggests common growth processes. However, kimberlite diatremes lithify faster than their basaltic counterparts, which leads to a higher probability of cognate floating reefs in kimberlites. Subsidence of floating reefs is a consequence of mass transfer during an eruption when root zone material is ejected upward through the diatreme, allowing for subsequent collapse of diatreme tephra to fill the void explosion chamber. Compaction of the volcaniclastic diatreme fill allows further subsidence. The downward movement of the root zone during pipe growth also initiates lateral erosion and enlargement of the overlying diatreme. The upward migration of arcuate tear-off fronts along the pipe walls causes isolated blocks of wall rock material to subside into the diatreme which widens the diatreme and, ultimately, the maar crater. Floating reefs provide important clues on the growth process of diatremes and can also have a controlling effect on the facies architecture. On that basis, they can influence diamond distribution in a primary diamond mine.

The debate revolves around the tectonic setting and petrogenesis of the Neoproterozoic Dokhan volcanics in the Egyptian Shield (ca.610–560 Ma by whole rock Rb–Sr and ca. 600–590 Ma by sensitive high-resolution ion microprobe SHRIMP U–Pb zircon), which formed during the transition from convergent to extensional, possibly after the collision of east and west Gondwana. The Dokhan volcanics suite in Egypt is undisturbed and geographically connected to immature clastic deposits from the Hammamat molasse-type sediments. These volcanics consist of basic, intermediate, and acidic rocks. The basic and intermediate Dokhan volcanics are represented by basalts and andesites, which are less common, while the acidic Dokhan volcanics are represented by rhyodacites, which are considered the main rock units encountered in the studied area. The geochemical values reveal broad trends of decreasing concentrations of compatible elements with MgO, Fe₂O₃, FeO, MnO, CaO, Co, Sr, and Zn, as well as increasing amounts of incompatible contents (Al₂O₃ and Na₂O) with increasing SiO₂ content. These geochemical characteristics are shared by Dokhan volcanics and associated Hammamat clastic sediments from Sinai and the Eastern Desert of Egypt. The geochemical behavior of Wadi Kid Dokhan volcanics is characterized by both orogenic arc-type and anorogenic within-plate tectonic environments. The magma types of Dokhan volcanics tend to have metaluminous to peraluminous, medium- to high-K calc-alkaline affinities. Meanwhile, the basaltic rocks are of tholeiitic origin. The lava flow of the medium- to high-K calc-alkaline post-collisional Dokhan volcanics shows a tectonomagmatic transition between the calc-alkaline arc-related magmatism and the alkaline anorogenic magmatism. The Dokhan volcanics in Sinai that were the focus of the inquiry exhibit a broad, evolving compositional range (rhyolite-basalt). A typical rhyolitic composition with an average silica concentration (69.5–70.9 wt%), an average andesitic content (57.3–58.6 wt%), and an average basaltic content (47.8 wt%) is also present in the Dokhan volcanics from the Eastern Desert of Egypt.

Seismic and mantle-xenolith data both show that cratons have the thickest, coldest lithosphere of the Earth’s tectonic environments. Yet, the quantitative depth distributions of temperature, seismic velocity and mass density (herein referred to as “density”) in cratonic lithosphere are uncertain, even on average across all cratons. Seismic surface-wave data offer abundant information on the thermal structure of the lithosphere at present, but seismic-velocity profiles in tomographic and other seismic models are highly non-unique at the relevant depth scale lengths of tens of kilometres. Here, we relate surface-wave measurements averaged over all cratons globally to the average physical properties of cratonic lithosphere using the recently developed methods of seismic thermography. The thermodynamic inversion of the Rayleigh and Love wave phase-velocity curves yields a model of the average structure of cratonic lithosphere, including the profiles of temperature, S- and P-wave seismic velocities, density and radial seismic anisotropy. Average depleted peridotite composition of the cratonic lithosphere was taken from the literature. Assuming 1290 °C as the temperature at which convection commences, which defines the bottom of the mechanical lithosphere, the best-fitting average depth of the cratonic lithosphere-asthenosphere boundary (LAB) is 228 km, with an uncertainty range of 211–242 km, as estimated using the model-space projection approach. The model fits the observed phase velocities very closely (misfits < 0.055%), while also matching the observed topography and surface heat flow. Assuming a lower LAB temperature results in a shallower LAB and a broader transition from the conductive lithospheric geotherm to the mantle adiabat, with a similar fit to the data. Our craton-average lithospheric model offers a useful reference for geophysical studies and for the joint analysis of geochemical and geophysical data. It confirms that cratonic lithosphere is, on average, isopycnic: cratonic and non-cratonic upper-mantle density profiles are very similar. A large proportion of published pressure–temperature measurements from mantle xenoliths is close to our craton-average lithospheric geotherm. Many of the measurements from below 150 km depth, however, show temperatures significantly higher than cratonic average, which offers useful evidence on the evolution of cratons and generation of kimberlites.

The Victor pipe (Attawapiskat kimberlite field, Superior Craton) and Star pipe (Fort à la Corne or FalC kimberlites, Sask Craton) host unconventional, lherzolite-associated diamond deposits. We used FTIR spectroscopy to quantify water contents in eclogite and pyroxenite mantle xenoliths from both localities in order to assess how Proterozoic tectonomagmatism, related to the Midcontinent Rift and Mackenzie Dike Swarm, affected the water inventory during craton reactivation. Fifteen eclogites and pyroxenites from Attawapiskat have C(H₂O) contents ranging from 269 to 883 (average 500) µg/g in clinopyroxene and 14 to 318 (average 70) µg/g in garnet. Clinopyroxene in seven eclogites and pyroxenites from FALC have C(H₂O) contents ranging from 270 to 502 (average 351) µg/g, and garnet from 14 to 70 (average 34) µg/g. Contrasting geochemical systematics for the two localities suggest differences in evolution. At Victor, crystal chemistry, in particular Al₂O₃ in clinopyroxene expressed as jadeite (Jd) component, appears to control water uptake for eclogites and pyroxenites. Highest H₂O contents are recorded in the more pristine jadeite-rich high-Ca eclogites, possibly pointing to an H₂O-rich environment typical of subduction zones. The more metasomatized samples (low Jd, high MgO contents, and enrichment in incompatible elements) show lower water contents, despite evidence for interaction with a comparatively hydrous (> 3 wt% H₂O) agent plausibly linked to ca. 1.15 Ga tectonism associated with the Midcontinent Rift. We attribute this to crystal-chemical modifications, particularly the reduced jadeite component in clinopyroxene, which diminishes its capacity to incorporate water. It is also possible that water activity was lowered if the metasomatic melt contained significant amounts of CO₂, as suggested by high Zr/Hf in the samples with the lowest H₂O content, resulting in a drying effect. In contrast, at Star, metasomatism appears to have overwhelmed the crystal-chemical effect of mantle metasomatism by means of high H₂O content in the metasomatic agent, as the high-Mg metasomatized samples show evidence for the greatest addition of water. It is noteworthy that in the Sask craton, where diamond formation has been linked to melt metasomatism and refertilization, the single diamond-bearing eclogite available for study has the lowest H₂O content among metasomatized eclogites. Although more data are needed, this may indicate that, if diamond formation was melt-mediated, it is more efficient at low H₂O activity that can ensue in the presence of CO₂.

The Oxford Lake—Knee Lake greenstone belt, northwestern Superior Craton, Canada hosts an unconventional diamond occurrence within Neoarchean ultramafic volcaniclastic rocks. Unusual features of the diamond occurrence include its ancient emplacement age (2.73 Ga), nature of the host rocks and high microdiamond counts (> 11,500 recovered from 1800 kg of drill core). The diamonds show unusual physical characteristics relative to typical kimberlite-hosted diamonds, having small dimensions, a restricted size range, prevalent cuboid primary shapes and low levels of nitrogen aggregation. These characteristics, together with a statistically significant correlation between shape and nitrogen aggregation, point to a single, rapid growth event in a cool mantle substrate for most of the Knee Lake diamonds, driven by C-oversaturation of the growth medium. Pressure–temperature estimates of diamond formation from a non-touching garnet-olivine inclusion pair indicate the lithospheric nature of the diamonds and formation at conditions equivalent to a modern-day reference model geotherm of 38–39 mW/m². This lithospheric column also contained eclogite, indicated by an omphacite inclusion in one diamond. The diamonds resided for a short time in a cool lithospheric mantle environment, possibly lower than 1050 °C at 5.5 GPa. These cool P–T conditions are difficult to achieve within > 150 km thick lithosphere without invoking slab stacking, a process supported by the C isotope composition of some of the diamonds. The Knee Lake diamond suite likely records cool transient thermal conditions inherited from subduction stacking of lithospheric slabs to form the lithosphere of the western Superior Craton.